It can take a considerable amount of time to download and display an image using a low-speed connection such as a telephone line or a cellular phone connection. For example, it can take 60 seconds to transmit an 800-by-600 pixel image with 24 bits per pixel over a standard telephone line. This transmission time presents problems because users typically expect an image to be transmitted and displayed within 10 seconds. If the image is not displayed within that time, then users may become dissatisfied with the delay and even abort the transmission.
Progressive image transmission (“PIT”) techniques have been used to help overcome this long delay before a user can view an image. PIT techniques start by transmitting the image in low resolution followed by increasingly higher resolutions. Since the image in low resolution can be transmitted much faster than in high resolution, the image can be displayed to the user more quickly, albeit in a lower resolution. A user can abort transmission of the image in high resolution based on viewing the image in low resolution because it may be not of interest, rather than having to wait until the image in high resolution is displayed.
With such progressive image transmission techniques, however, users still need to wait until the entire image in low resolution is transmitted and displayed before a decision can be made as to whether the image is of interest. Some progressive image transmission techniques attempt to identify important areas of an image and transmit those important areas before transmitting less important areas. Such techniques typically use a minimal distortion principle to identify areas of high distortion and transmit those areas first. Relying on the minimal distortion principle, however, does not necessarily provide the best viewing experience when an image is transmitted progressively. The importance based on the minimal distortion principle does not correspond very well to what people perceive as important. In addition, since people perceive only part of an image to be important, the minimal distortion principle may be applied to unimportant parts.
It would be desirable to have a progressive image transmission technique that would automatically identify areas of an image that people would perceive as important and base progressive image transmission on that importance.